[Elecraft] Field Day experience with K3

[Edward R. Cole]

Dave, ab9ca/4 wrote:

This isn't right. The capacity of the battery in watts is 110x12 or
about 1320WHr. If the load is 122w/hr the battery should be totally
exhausted after about 11 hours.
----------
yep, I confused AH with WH, so that does not explain the problems the 
original writer had.
Another way to analyze would be using just the current load:  2x17a = 
34a x 30% = 10.2amp

Then 110 A-H/10.2 A = 10.8 hours (theoretically)

But battery discharge curves display the actual battery operating 
voltage one can expect to see.  so one would probably reach an 
unworkable voltage sooner than 10.8 hours.  The only good analysis 
would be if you have the discharge curve for the battery.  These 
usually assume a <10% load (e.g. < 11 A).  Using the discharge curve 
you can predict battery life under load.

I used a 30% duty cycle which provides the time weighting factor 
(integration factor). You can argue that in FD operations the duty 
cycle could be something other than 30%.  I assumed that one does a 
lot more calling in FD than normal assumed Tx/Rx ratios that 
commercial radio industry uses (10%).

In my professional observations of radio using backup battery power 
the useful life never approaches the theoretical expected battery 
life.  Most of the time the system becomes unusable in about half the 
time expected from a battery bank.  This is on a properly floated 
battery bank with periodic equalization (hams normally do not do this).

Theoreticals are only a good starting point as they assume fully 
charged new batteries (rarely is either true).

The 120AH solar charging system "theoretically" provides 10A at 12V 
which would lead one to believe it would supply the operating load of 
two radios.  I rarely have seen solar panels output their full rated 
power.  In the brightest sun it is normally running about 80% of label ratings.

So there was something else occurring to cause the voltage to sag to 
10v in the original posters operation.






73, Ed - KL7UW, WD2XSH/45
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